Chapter 5 · Signs of a chemical changeTextbook pages 95–97Figs 5.1–5.2 · Tables 5.1, 5.2

Signs of a Chemical Change രാസമാറ്റം തിരിച്ചറിയാം

The chapter begins in the kitchen, where physical and chemical changes happen every day. On this page you will sort the changes of Table 5.1. Then you will watch the book's four test-tube experiments on a virtual bench and find the signs that tell us a chemical change has happened: a colour change (നിറവ്യത്യാസം), a gas (വാതകം), a precipitate (അവക്ഷിപ്തം) and a smell (ഗന്ധം). The page ends with quicklime and water, a reaction that makes the beaker hot.

📖 What you need (pages 1–2 of this guide)

Physical and chemical changes, reactants and products, ↑ and ↓ in equations (page 1).
Particles are always moving (page 2) – this is how a smell reaches your nose.

1. The kitchen is a laboratory (page 95)

നമ്മുടെ വീടുകളിലെ അടുക്കള ഒരു രസതന്ത്രശാലയാണെന്ന് നിങ്ങൾക്കറിയാമോ? നിരവധി ഭൗതിക, രാസമാറ്റങ്ങൾ അവിടെ സംഭവിക്കുന്നുണ്ട്. – Did you know that the kitchen in our homes is a chemistry laboratory? Many physical and chemical changes take place there.

In the book's opening picture a girl walks into the kitchen and asks, “ഇന്നെന്താ കറി? നല്ല മണം വരുന്നുണ്ടല്ലോ?” – “What's the curry today? There's a lovely smell!” That smell is itself a clue: while the onions and spices fry, new substances form and their molecules spread through the air to her nose.

🍳 Kitchen hunt – tap a change to find out whether it is physical or chemical

Think (page 95). Name some physical changes and some chemical changes that happen in your kitchen.

Physical changes: water boiling, ice melting, sugar or salt dissolving, coconut oil melting, cutting and grinding vegetables, steam condensing on the lid.
Chemical changes: LPG or firewood burning, rice and vegetables cooking, onions browning while frying, milk turning into curd, dosa or idli batter fermenting, a cut apple or brinjal turning brown, milk going sour.

2. Table 5.1 – physical or chemical? (page 96)

ചുവടെ നൽകിയിരിക്കുന്ന പ്രക്രിയകളെ ഭൗതികമാറ്റങ്ങൾ, രാസമാറ്റങ്ങൾ എന്നിങ്ങനെ വർഗീകരിക്കുക. – Classify the processes given below as physical changes or chemical changes. Type physical or chemical.

✍ Table 5.1

Ice melting – physical (still water). Firewood burning – chemical (ash, CO₂ and water vapour form, with heat and light). Milk turning into curd – chemical (new substances with a new taste). Sugar dissolving in water – physical (the sugar can be got back). Water boiling – physical (steam is still H₂O).

3. Chemical reactions are everywhere (page 96)

രാസപ്രവർത്തനങ്ങൾ സാങ്കേതികവിദ്യയുടെയും സംസ്കാരത്തിന്റെയും നിത്യജീവിതത്തിന്റെയും അവിഭാജ്യ ഘടകമാണ്. – Chemical reactions are an inseparable part of technology, culture and daily life. Burning fuels, smelting iron (getting the metal from its ore), making glass and pottery, and making wine and cheese have been known for thousands of years. Many chemical reactions also take place in the complex processes on the Earth's surface, in the atmosphere, in the oceans and in living things.

4. What changes come with a chemical reaction? (pages 96–97)

ഒന്നോ അതിലധികമോ പുതിയ പദാർഥങ്ങൾ ഉണ്ടാകുന്ന തരത്തിൽ ദ്രവ്യത്തിനുണ്ടാകുന്ന മാറ്റമാണ് രാസമാറ്റമെന്നു നിങ്ങൾ മനസിലാക്കിയിട്ടുണ്ടല്ലോ? കൂടാതെ രാസമാറ്റത്തിനോടനുബന്ധിച്ച് ഊർജമാറ്റവും സംഭവിക്കുന്നു. – You have learnt that a chemical change (രാസമാറ്റം) is a change in matter in which one or more new substances are formed. An energy change also takes place with it. Let us test, through a few experiments, what changes take place during chemical reactions.

⚠ Safety first. Experiment 1 uses a flame and experiment 2 uses dilute acid: in school they are done by or under the close supervision of your teacher, with goggles. Experiments 3 and 4 use poisonous substances and are teacher demonstrations only. Here all four are safe virtual demonstrations. Never try any of them at home.

🧑‍🏫 Virtual bench · four experiments, four signs

Experiment 1 · heating sugar

ഒരു സ്പാച്ചുലയിൽ കുറച്ച് പഞ്ചസാര (C₁₂H₂₂O₁₁) എടുത്ത് ബൺസൺ ബർണറിൽ ചൂടാക്കുക. ലഭിച്ച ഉൽപന്നത്തിന്റെ നിറം എന്താണ്? A little sugar (C₁₂H₂₂O₁₁) is taken on a spatula and heated in a Bunsen burner flame. What is the colour of the product?

The white sugar melts, turns brown and finally black. The black substance is mainly carbon (charring), and water vapour is given off. Sugar has changed into new substances, so this is a chemical change, recognised by the change of colour (നിറവ്യത്യാസം).
Go further: on strong heating, C₁₂H₂₂O₁₁ → 12C + 11H₂O. (Caramel, the brown stage, is a mixture of new substances too.)

Experiment 2 · sodium carbonate and dilute hydrochloric acid (Fig 5.1)

ഒരു ടെസ്റ്റ് ട്യൂബിൽ കുറച്ച് സോഡിയം കാർബണേറ്റ് (Na₂CO₃) എടുക്കുക. അതിലേക്ക് 2-3 mL നേർപ്പിച്ച ഹൈഡ്രോക്ലോറിക് ആസിഡ് (HCl) ചേർക്കുക (ചിത്രം 5.1). നിങ്ങളുടെ നിരീക്ഷണം എന്താണ്? A little sodium carbonate (Na₂CO₃) is taken in a test tube and 2–3 mL of dilute hydrochloric acid (HCl) is added (Fig 5.1). What do you observe?

The mixture fizzes: lots of bubbles rise and froth up (effervescence). A gas (വാതകം) is given off – it is carbon dioxide, which would turn lime water milky. The white powder disappears as it reacts.
Go further: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

Experiment 3 · lead acetate and potassium iodide (Fig 5.2)

⚠ Teacher demonstration only. Lead compounds such as lead acetate and lead iodide are poisonous. Students must not handle them. Watch from a safe distance; never touch or taste anything, and wash your hands after the practical. Your teacher will dispose of the waste safely.

ഒരു ടെസ്റ്റ് ട്യൂബിലെ 2-3 mL ലെഡ് അസറ്റേറ്റ് (Pb(CH₃COO)₂) ലായനിയിലേക്ക് 2-3 mL പൊട്ടാസ്യം അയോഡൈഡ് (KI) ലായനി ചേർക്കുക (ചിത്രം 5.2). ലഭിച്ച ഉൽപന്നം ടെസ്റ്റ് ട്യൂബിൽ ഏതു ഭാഗത്താണ് കാണപ്പെടുന്നത്? അതിന്റെ നിറം എന്താണ്? 2–3 mL of potassium iodide (KI) solution is added to 2–3 mL of lead acetate (Pb(CH₃COO)₂) solution in a test tube (Fig 5.2). In which part of the test tube is the product seen? What is its colour?

Two colourless solutions give a bright yellow solid at once. The solid slowly settles at the bottom of the test tube. An insoluble solid formed in a solution like this is a precipitate (അവക്ഷിപ്തം); it is lead iodide, PbI₂.
Go further: Pb(CH₃COO)₂ + 2KI → PbI₂↓ + 2CH₃COOK

Experiment 4 · iron sulphide and dilute hydrochloric acid

⚠ Teacher demonstration only – in a fume cupboard. The gas formed, hydrogen sulphide, is poisonous even though we can smell very small amounts of it. Never put your nose over a test tube to smell a gas. If your teacher asks you to notice a smell, stay at a distance; your teacher may show the safe way of wafting (page 8).

ഒരു ടെസ്റ്റ് ട്യൂബിൽ കുറച്ച് അയൺ സൾഫൈഡ് (FeS) എടുക്കുക. അതിലേക്ക് 2-3 mL നേർപ്പിച്ച ഹൈഡ്രോക്ലോറിക് ആസിഡ് ചേർത്ത് ചൂടാക്കുക. ഉണ്ടാകുന്ന ഗന്ധം പരിചിതമാണോ? A little iron sulphide (FeS) is taken in a test tube, 2–3 mL of dilute hydrochloric acid is added and it is warmed. Is the smell produced familiar?

Yes – it is the smell of rotten eggs. It comes from a new gas, hydrogen sulphide (H₂S). A new smell (ഗന്ധം) is a sign of chemical change.
Go further: FeS + 2HCl → FeCl₂ + H₂S↑

5. Table 5.2 – how was the chemical change recognised? (page 97)

മുകളിൽ കൊടുത്തിരിക്കുന്ന പ്രവർത്തനങ്ങളുടെ അടിസ്ഥാനത്തിൽ പട്ടിക പൂർത്തിയാക്കുക. – Complete the table on the basis of the activities above. Write in your own words (for example “a gas is given off”); the checker looks for the key word.

✍ Table 5.2 – രാസമാറ്റം തിരിച്ചറിഞ്ഞ വിധം (how the chemical change was identified)

⭐ Signs of a chemical change (page 97)

രാസപ്രവർത്തനങ്ങളുടെ ഫലമായി നിറവ്യത്യാസത്തിന്റെയോ, സ്വതന്ത്രമാക്കപ്പെടുന്ന വാതകത്തിന്റെയോ, അനുഭവപ്പെടുന്ന ഗന്ധത്തിന്റെയോ, അവക്ഷിപ്തപ്പെടുന്ന (precipitation) ഉൽപന്നങ്ങളുടെയോ അടിസ്ഥാനത്തിൽ രാസമാറ്റം തിരിച്ചറിയാനാകുമെന്ന് മനസ്സിലാക്കാം.

A chemical change can be recognised by a colour change, a gas given off, a smell, or a product that precipitates (അവക്ഷിപ്തപ്പെടുക). A change in temperature (heat given out or taken in) or light is another sign, as the next experiment shows.

🤔 A sign is a clue, not a proof

Boiling water also gives bubbles, and a gas escaping does not always mean a reaction. Always ask the real question: has a new substance formed? In experiment 2 the bubbles are a new gas (CO₂) that was not there before; in boiling water the bubbles are just water vapour.

6. Quicklime and water – the beaker gets hot (page 97)

ഉണ്ടാകുന്ന ഉൽപന്നത്തിന്റെ അടിസ്ഥാനത്തിൽ രാസപ്രവർത്തനങ്ങളെ എങ്ങനെയെല്ലാം വർഗീകരിക്കാൻ കഴിയുമെന്ന് നോക്കാം. – Now let us see how chemical reactions can be classified on the basis of the products formed. The book begins with a reaction you may have seen when walls are whitewashed.

ഒരു ബിക്കറിൽ കുറച്ച് നീറ്റുകക്ക (CaO) എടുക്കുക. അതിലേക്ക് കുറച്ച് ജലം ഒഴിക്കുക. ബിക്കറിന്റെ പുറത്ത് തൊട്ടു നോക്കുക. – A little quicklime (നീറ്റുകക്ക, calcium oxide, CaO) is taken in a beaker and a little water is poured on it. Touch the outside of the beaker.

⚠ Quicklime is caustic: it burns skin and eyes, and with water it can get very hot and spit. Use only a small amount under your teacher's supervision, wear goggles, never touch the lumps, and touch only the outside of the beaker, carefully.

🧪 Virtual lab · quicklime + water

The thermometer numbers are illustrative: how hot it gets depends on how much quicklime and water are used.

Question (page 97). എന്താണ് നിരീക്ഷണം? What do you observe?

The beaker becomes hot. The lumps hiss, crack and crumble into a white powder or paste, and some steam may rise. ഇവിടെ കാൽസ്യം ഓക്സൈഡ്, ജലവുമായി പ്രവർത്തിക്കുമ്പോൾ ലഭിക്കുന്നത് കാൽസ്യം ഹൈഡ്രോക്സൈഡ് (Ca(OH)₂) എന്ന സംയുക്തമാണ്. കൂടാതെ ഇതൊരു താപമോചക (exothermic) പ്രവർത്തനവുമാണ്. Calcium oxide reacts with water to form a new compound, calcium hydroxide (Ca(OH)₂, slaked lime). The reaction gives out heat, so it is an exothermic reaction (താപമോചക പ്രവർത്തനം):

CaO + H₂O → Ca(OH)₂ + heat

Two substances have joined to give one product. This is a combination reaction – the first reaction type, on the next page.

⭐ Key ideas

Table 5.1: ice melting, sugar dissolving and water boiling are physical; firewood burning and milk turning into curd are chemical.
Signs of a chemical change: colour change (sugar turns black), gas (Na₂CO₃ + HCl fizzes), precipitate (yellow lead iodide at the bottom), smell (rotten eggs from FeS + HCl), and heat (quicklime + water).
CaO + H₂O → Ca(OH)₂ + heat is an exothermic (താപമോചക) reaction.
Lead compounds and hydrogen sulphide are poisonous: teacher demonstrations only.

⚠️ Common mistakes

“Sugar dissolving is a chemical change because the sugar disappears” ✗ – it is still sugar; evaporate the water and it is back.
“Any bubbles mean a reaction” ✗ – boiling water bubbles too. Ask whether a new substance has formed.
“The precipitate floats at the top” ✗ – a precipitate is a solid that settles at the bottom.
Smelling a gas straight from the tube ✗ – never; some gases, like H₂S, are poisonous.

🌐 Explore online needs internet · opens in a new tab

📄 Exothermic reaction Wikipedia
Reactions that give out heat, like quicklime with water.
📄 Chemical decomposition Wikipedia
One compound breaks into two or more substances.
📄 Single displacement reaction Wikipedia
One element takes the place of another in a compound.
📄 Salt metathesis (double decomposition) reaction Wikipedia
Two compounds swap their ions.
📄 Precipitation (chemistry) Wikipedia
How an insoluble solid forms from two solutions.
📄 Airbag Wikipedia
Air bags fill with nitrogen from a very fast decomposition reaction.
📄 Thermite Wikipedia
Aluminium displaces iron from iron oxide – used to weld rails.

7. Check yourself

Next: Combination and decomposition reactions ▶